1. Field of the Invention
The present invention relates to a display device comprising an emissive element such as an electroluminescence element and thin film transistors.
2. Description of Related Art
Electroluminescence (EL) display devices using an electroluminescence element have recently attracted interest as potential replacements for devices such as CRT or LCD displays. For example, an EL display device having a thin film transistor (TFT) as a switching element for driving the EL element has been studied and developed.
FIG. 1 shows, in a plan view, one display pixel of a related organic EL display device. FIG. 2 is an equivalent circuit diagram of an organic EL display device corresponding to one display pixel. FIGS. 3A and 3B are cross sections taken along lines Axe2x80x94A and Bxe2x80x94B in FIG. 1, respectively.
Referring to FIGS. 1 and 2, a display pixel is formed in a region enclosed by a gate signal line 151 and a drain signal line 152. Around the intersection of both signal lines is formed a first TFT 130 as a switching TFT. A source 131s of the first TFT 130 also functions as a storage capacitor electrode 155 constituting a capacitor between the source 131s and a storage capacitor electrode 154 which will be described later, and is connected to a gate 142 of a second TFT 140 which drives an organic EL element. A source 141s of the second TFT 140 is connected to an anode 161 of the organic EL element-while a drain 141d is connected to a power supply line 153 for driving the organic EL element.
The storage capacitor electrode 154 is disposed in parallel to the gate signal line 151 in the vicinity of the first TFT. The storage capacitor electrode 154 comprises chromium or the like and forms a capacitor 170 for accumulating charges between the storage capacitor electrode 154 and the storage capacitor electrode 155 connected to the source 131s of the first TFT 130 via a gate insulating film 112. The storage capacitor 170 is provided so as to hold a voltage to be applied to the gate 142 of the second TFT 140.
The first TFT 130, which is a switching TFT, will now be described.
Referring to FIG. 3A, on an insulating substrate 110 comprising a quartz glass, non-alkali glass, or the like, are provided the gate signal line 151 including gate electrodes 132 and comprising a refractory metal with a high melting point such as chromium (Cr), molybdenum (Mo), or the like, and the storage capacitor electrode 154.
Subsequently, the gate insulating film 112, and an active layer 131 comprising a poly-silicon (hereinafter referred to as xe2x80x9cp-Sixe2x80x9d) film are sequentially formed, such that the active layer 131 has a so-called LDD (Lightly Doped Drain) structure. More specifically, low concentration regions 131LD are formed at both sides of the pair of channel regions 131c opposing to gate electrodes 132, and a source 131s and a drain 131d of high concentration regions are further formed to the outsides of the low concentration regions.
Further, over the entire surface covering the gate insulating film 112, the active layer 131, and stopper insulating films 114, an interlayer insulating film 115 having a multi-layer structure is provided, and a metal such as Al or the like is used to fill a contact hole formed in the position in the interlayer insulating film 115 corresponding to the drain 131d to form a drain electrode 116. Then, a planarization insulating layer 117 comprising an organic resin is provided on the entire surface to planarize the surface.
Next, the second TFT 140, which is an organic EL element driving TFT, will be described.
Referring to FIG. 3B, on the insulating substrate 110 comprising a quartz glass, non-alkali glass, or the like, the gate electrodes 142 each comprising a refractory metal (metal having a high melting point) such as Cr, Mo, or the like are formed, and the gate insulating film 112 and an active layer 141 comprising a p-Si film are sequentially formed thereon. In the active layer 141, channels 141c which are intrinsic or substantially intrinsic are formed at the positions above the respective gate electrodes 142, and the source 141s and the drain 141d are formed by doping p-type impurities at each side of the channel pair to thereby constitute a p-type channel TFT.
The interlayer insulating film 115 of a multi-layer structure is then provided over the entire surface on the gate insulating film 112 and the active layer 141. A contact hole provided in the interlayer insulating film 115 so as to correspond to the drain 141d is filled with a metal such as Al to form the power supply line 153 connected to the power supply 150. Further, the planarization insulating film 117 comprising an organic resin to planarize the surface is provided over the entire surface. On the planarization insulating film 117, a transparent electrode, in this case the anode 161 of the organic EL element, which comprises ITO is provided to make contact with the source 141s via a contact hole formed at the position of the planarization insulating film 117 and the interlayer insulating film 115 corresponding to the source 141s. 
The organic EL element 160 comprises the anode 161 comprising ITO (Indium Tin Oxide) or the like and connected to the source 141s of the second TFT 140, an emissive element layer 166 comprising an organic compound, and a cathode 167 using magnesium-indium alloy, formed in that order. In the organic EL element 160, holes injected from the anode 161 and electrons injected from the cathode 167 are recombined inside the emissive element layer 166 to excite organic molecules forming the emissive element layer 166 for causing exciton. In the process of radiation and deactivation by the exciton, the emissive element layer 166 produces light which is emitted from the transparent anode 161 through the transparent insulating substrate 110.
As shown in FIG. 3B, the anode 161 forms an individual pattern corresponding to each display pixel, and the emissive element layer 166 which is somewhat larger than the anode 161 is formed so as to entirely cover the anode 161. On the other hand, the cathode 167, which can be commonly used electrically, is formed as a common electrode.
In each pixel, charges applied from the source 131s of the selected first TFT 130 are accumulated and held in the storage capacitor 170 and are also applied to the gates 142 of the second TFT 140. A current in accordance with a voltage applied to the gates 142 is applied from the power supply 150 to the organic EL element which then emits light.
In the EL display device, however, if the aperture ratio of the display pixel is small, light is irradiated from a small area of the emissive layer of the organic EL element, which results in an extremely dark display.
To cope with the foregoing disadvantage, increase of the aperture ratio may be considered. However, to increase the aperture ratio, the area of a storage capacitor which constitutes the non-emissive region within a display pixel must be decreased, which leads to problems. Especially, decrease in the storage capacitor area will reduce the storage capacity, which in turn makes it impossible for only the storage capacitor 170 to sufficiently hold the drain signal supplied from the first TFT 130 until next time the first TFT 130 is selected. As a result, the gates 142 of the second TFT 140 cannot be put into a sufficient on-state to cause the organic EL element to emit light for a sufficient period to provide a bright display.
The present invention was conceived in view of the above described problems of the related art and aims to provide an EL display device which can hold sufficient charge to produce a bright display without decreasing the aperture ratio.
In accordance with one aspect of the present invention, there is provided an electroluminescence display device comprising an electroluminescence element disposed on a substrate and having first and second electrodes and an emissive layer disposed between the first and second electrodes; a first thin film transistor in which a gate electrode is connected to a gate line and a first electrode area is connected to a data line; a second thin film transistor in which a gate electrode is connected to a second electrode area of said first thin film transistor, a first electrode area is connected to a power supply to said electroluminescence element, and a second electrode area is connected to the electroluminescence element; and first and second storage capacitors, one storage capacitor electrode of each of which is connected to the second electrode area of said first thin film transistor and to the gate electrode of said second thin film transistor.
In another aspect of the present invention, there is provided an electroluminescence display device comprising an electrolumincescence element having an anode, a cathode, and an emissive layer interposed between the anode and the cathode; a first thin film transistor in which a gate electrode is connected to a gate line and a first electrode area is connected to a data line; a second thin film transistor in which a gate electrode is connected to a second electrode area of said first thin film transistor, a first electrode area is connected to a power supply for said electroluminescence element, and a second electrode area is connected to the electroluminescence element; a first storage capacitor including a first storage capacitor electrode and a second storage capacitor electrode which is connected to the second electrode area of said first thin film transistor and to the gate electrode of said second thin film transistor; and a second storage capacitor including said second storage capacitor electrode and a third storage capacitor electrode connected to a power supply line for supplying a power from said power supply to said electroluminescence element.
In accordance with still another aspect of the present invention, there is provided an light emissive display device comprising an emissive element disposed on a substrate and having first and second electrodes and an emissive layer provided between the first and second electrodes; a first thin film transistor in which a gate electrode is connected to a gate line and a first electrode area is connected to a data line; a second thin film transistor in which a gate electrode is connected to a second electrode area of said first thin film transistor, a first electrode area is connected to a power supply for said emissive element, and a second electrode area is connected to the emissive element; and first and second storage capacitors each having one storage capacitor electrode connected to the second electrode area of said first thin film transistor and to the gate electrode of said second thin film transistor.
In accordance with another aspect of the present invention, said first storage capacitor is formed by arranging the first storage capacitor electrode and the second storage capacitor electrode connected to the second electrode area of the first thin film transistor and to the gate electrode of the second thin film transistor such that they are opposed to each other via an insulating layer, and said second storage capacitor is formed by arranging said second storage capacitor electrode and the third storage capacitor electrode such that they are opposed to each other via an insulating layer.
In accordance with another aspect of the present invention, said first and second storage capacitor share one electrode connected to both the second electrode area of said first thin film transistor and to the gate electrode of the second thin film transistor.
In accordance with another aspect of the present invention, said first storage capacitor is formed by arranging the first storage capacitor electrode and the second storage capacitor electrode connected to the second electrode area of the first thin film transistor and to the gate electrode of the second thin film transistor such that they are opposed to each other via an insulating layer, and said second storage capacitor is disposed so as to superpose said first storage capacitor and is formed by arranging said second storage capacitor electrode and the third storage capacitor electrode such that they are opposed to each other via an insulating layer.
The first and second storage capacitors as described above can easily increase the capacity of each pixel without reducing the aperture ratio and can securely hold a voltage applied to the gate electrode of the second thin film transistor for a sufficient period. As a result, a desired power (e.g. current) can be securely supplied from the power supply to the emissive element, such as an organic EL element, via the second thin film to thereby implement a display device providing bright display with high image quality.
In accordance with another aspect of the present invention, the other storage capacitor electrode of said second storage capacitor or the third storage capacitor electrode is integrally formed with the power supply line for supplying a power from said power supply to the first electrode area of the second thin film transistor from said power supply.
In accordance with still another aspect of the present invention, a predetermined direct current voltage is applied to the first storage capacitor electrode.
By thus forming the other storage capacitor electrode of the second storage capacitor to be integrally formed with the power supply line, it is possible to accurately dispose said other storage capacitor electrode at a predetermined position, for example, a position superimposing the first storage capacitor, only with a change in the pattern of the power supply line. This eliminates any need for extra processes for forming the second storage capacitor, and improved display quality can be obtained without raising the manufacturing cost.
In accordance with still another aspect of the present invention, the emissive layer may use an organic compound. An emissive layer using an organic compound is very advantageous in a color display device because of the possible variations of emissive colors and a wide selection of possible materials.